This invention relates to an air-impervious flexible blanket suitable for overlaying sheets of photographic material on contact exposure apparatus. More specifically, this invention relates to the method and means for producing operable air flow channels on the underside of the flexible blanket using printing techniques.
In contact exposure applications, vacuum is used to establish an even and close contact between the original and the photosensitive materials. These photo materials may be film, paper, metal or plastic plates, silk screens, or the like. Often the exposure requires the sandwiching of several layers of such materials.
To facilitate the vacuum contact system, flexible blankets are used to cover the materials. The border of these blankets, usually made of rubber, seal against the glass surface through which the exposure is made. The blanket itself includes one or more evacuation valves in the corners. Once the vacuum pump is activated, the air above, below and in-between the materials and the glass is evacuated to establish good contact registration.
One disadvantage encountered in these known schemes is that the vacuum being drawn from the corners and the sides first evacuates the air closest to the valves. At these points, usually along the sides, the materials bind and seal quickly, trapping air pockets in the center area that take a long time to evacuate completely. To overcome this problem it is common that the vacuum is activated in two stages with a pause after the initial draw to just a few inches of mercury. This reduces the problem of binding when the full vacuum is drawn to 23-26 inches of mercury. This does, however, increase the time required per operation. Techniques have been developed to squeeze out entrapped pockets of air, as described in the literature (See, for example, U.S. Pat. No. 4,504,142).
One known scheme for keeping the air passage open to the middle of the frame is through a molded pattern on the blanket. However, this scheme does not help evacuation between the materials, and it also raises the total volume to be evacuated and, worst of all, it often causes a distinct pattern to be embedded in the exposed surface by the pressure points in the pattern. Molding such blankets usually requires a substantial tooling charge in large sizes. In order to avoid this cost and due to the many varied blanket sizes used, most manufacturers produce pre molded material in continuous sheets, and then cut the sheet and either grind the outer edges smooth or vulcanize a straight molding along the edges that can vacuum seal to the glass plate. Because these blankets are often exposed to high intensity ultra-violet light rays, the rubber deteriorates requiring periodic replacement at great cost.
In accordance with the present invention there is provided a method and means by which a suitable pattern of air passages can be produced on standard stock of flexible sheet blanket material. More specifically, the present invention uses standard printing techniques to emboss the desired pattern on the underside surface of a rubber or plastic or other elastomeric sheet. The pattern may be silk screened or otherwise printed onto one surface of the blanket using curable liquid material to provide spacing between blanket and sheet material to facilitate efficient evacuation of air. In another embodiment that starts with conventional, patterned blanket material, the outer edge may be silk-screened or otherwise printed to fill the pattern with curable liquid material to provide a smooth perimeter edge which will establish an air-tight seal, and which therefore obviates the need to grind the edge smooth. A wide variety of patterns can be developed for different applications including patterns that are elevated or wider in the middle than at the edges. This is particularly advantageous with certain photographic sheets that are sensitive to the point pressures of blanket patterns since screened or printed patterns produced according to the present invention have smooth edges and can be applied to a soft, elastomeric blanket. The curable liquid material that is screened or otherwise printed onto the blanket may be inks or paints with sufficient plasticizers and additives to retain a cured height of a few thousandths of an inch.
In accordance with the present invention, an improved vacuum system evacuates residual air from beneath this blanket through vias or channels that are located beyond the perimeter of the glass exposure plate.